This invention relates to the field of solvent extraction. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
Effluent streams containing elements of the actinide series and the lanthanide series of the periodic table are generated in certain nuclear energy applications, such as the processing of waste from a light water nuclear reactor. The actinides and lanthanides are usually in a trivalent state and in an acidic aqueous solution. It is desirable to separate the two classes of elements for a variety of reasons. For example, the half-lives of lanthanides are much shorter than those of actinides; therefore, the quantity of waste which must be stored for a very long time can be reduced by removing lanthanides from the waste. Also, actinides may be used in applications which require a radioactive source. Further, it is necessary to characterize radioactive effluent streams and separation of actinides from lanthanides is a first step in quantitative analysis of an effluent stream.
The separation of trivalent actinides from trivalent lanthanides is one of the more difficult problems in separation science. These elements possess few of the differences in physical and chemical characteristics necessary to effect separation of the groups from each other. For example, the separation is difficult because trivalent lanthanide atoms and trivalent actinide atoms have similar chemical properties, their ionic radii are similar in size, and complexes of these elements exhibit a strong ionic character.
Michelle Bonnin et al., "Process for Separating the Actinides and Lanthanides Present in the Trivalent State in an Acid Aqueous Solution," U.S. Pat. No. 4,496,523, Jan. 29, 1985, teaches separation of actinides from lanthanides by extraction using an organic solvent comprising a first extracting agent formed by an organic bonding agent having an electron-donor nitrogen atom and a second extracting agent formed by an acid organosoluble organic compound able to exchange its hydrogen ions for metal ions or by a metallic salt of such a compound.
Richard Fitoussi et al., "Method for Separating Actinides from Lanthanides in an Acidic Aqueous Solution," U.S. Pat. No. 4,461,747, July 24, 1984, teaches the separation using an organic solvent comprising a system of extractants constituted by an acidic organophosphorated compound containing at least one electron-donor sulfur atom, such as di-2-ethylhexyl-dithio-phosphoric acid, and by a neutral organophosphorated compound containing at least one electron-donor oxygen atom, such as trioctylphosphine oxide.
Herve Hubert et al., "Process for Recovering Actinides and/or Lanthanides," U.S. Pat. No. 4,572,802, Feb. 25, 1986, teaches a process for recovering both lanthanides and actinides from an aqueous acid solution, but which does not separate actinides from lanthanides.